The present invention relates generally to a pulse combustor for providing an oscillating stream of hot combustion products, and more particularly to a pulse combustor arrangement in which the frequency and amplitude of the oscillations are selectively controllable.
Pulse combustors have been found to be valuable as mechanisms for providing sonic streams of hot gases which can be utilized in various applications such as drying, smelting, water heating, and vortex heaters. Also, the sonic energy or sound waves accompanying the hot gases as provided by the cyclic combustion process inherent in pulse combustors provides for efficient slurry atomization and the dispersion of fluids, liquids, and powders.
Pulse combustors are of relatively simple construction and generally comprise a combustion chamber with fuel and air supply systems and a spark plug for effecting the ignition of the fuel-air mixture. An exhaust nozzle, which may be acoustically tuned, is attached to the combustion chamber through a tapered flange for conveying the sonic streams of the hot combustion gases to a point of use. The combustion process is initiated when a charge of air and fuel is introduced into the combustion chamber and ignited by the spark plug. The burning of the gases occurs rapidly and causes the pressure of the gases to rise in the combustion chamber. This pressure increase results in the combustion gases quickly exiting the combustion chamber through the exhaust opening at very high velocities. As the pressure falls in the combustion chamber after an explosive event, another charge of fuel and air is introduced into the combustion chamber to repeat the cycle. This cyclic combustion process is continuous with the rate of oscillation depending upon the particular geometry of the combustion chamber, fuel-air input rates, and the ignition system utilized. The rapid combustion of the fuel-air mixture provides a flow of hot gases from the combustor through the nozzle with these gases oscillating at various amplitudes and frequencies depending upon the operating parameters within the combustor.
Generally, pulse combustors have been found to be very versatile and can use almost any fuel capable of being delivered through a conduit such as liquid fuels including fuel oil, diesel oil, alcohols, as well as gaseous fuels such as natural gas and propane. Also, oil- or coal-water slurries may be used as the fuel in the combustors.
The pulse combustors as presently known include the valveless type where the air supply is introduced into the combustion chamber through self aspiration via an essentially open front end, the valved type which utilize a flapper valve which opens and closes during each cycle to provide the combustion supporting air, and another type of valveless pulse combustor where the front end of the combustor is closed and the air is introduced into the combustion chamber in a suitable manner from a pressurized source. The operation of the combustors using a compressed air supply arrangement is advantageous since it has the better control features of the aforementioned types of combustors particularly due to the fact that the fuel-air ratios and the throughput or flow time for the reactants may be selectively varied to provide a greater measure of control over the frequency and amplitude of the combustion oscillations. Such control over the frequency and amplitude of the oscillations is desirable where the high-velocity pulsations from the combustor are to be utilized for slurry atomization or as a particulate dispersing mechanism. In such instances high amplitude oscillations at high frequencies are believed to be particularly desirable.
However, there are some problems attendant with providing and maintaining a suitable level of control over the amplitude and frequencies provided by the oscillating combustors as presently known. These problems or shortcomings are primarily due to the discovery that heat transfer within the combustion zone was found to be significant in the control of the combustion oscillation amplitude and frequency. Most notably is the temperature of the combustor walls in that as the wall temperature increases during the operation, the oscillating frequency also increases while the oscillating amplitude decreases. In fact, the frequency continually increases to such a point with increasing wall temperatures that an essentially steady flame eventually replaces oscillations of any significance in the combustor.
With the operating frequency increasing with increasing wall temperatures, the combustor wall temperature and the oscillating pressure are directly related. At fixed throughputs or flow times of fuel and air, the pressure of the oscillations decreases with increasing wall temperature. Thus, any increase in wall temperature is also accompanied by an increase in frequency with the pressure or amplitude being inversely related to the frequency. In previous demonstrations of thermally induced pulse combustion devices, the higher wall temperatures produced pulses at higher frequencies and lower amplitudes. It is important to realize that in order to generate oscillations in a pulse combustor, it is required that the flame be nearly extinguished within the combustor after each explosion so as to provide a defined, sharp pulse or pressure spike in the oscillation. The more rapid the extinguishing of the flame after the thermal explosion the greater the amplitude and also frequency of the oscillation.
Efforts to regulate the oscillating frequency of pulse combustors included varying the air-fuel ratio or equivalence ratio and increasing the throughput by using lower flow times where the fuel and air are introduced into the combustion chamber from pressurized sources. Convective heat transfer from the high temperature walls promoted reignition of the fuel-air mixture so that it was expected that an increase in the throughput would effect the heat transfer rate since the higher mass flow would require more heat to raise the temperature to the ignition temperature. Thus, given a fixed heat transfer area such as provided by the combustor walls and assuming that the local reactions near the spark plug would not be significantly affected by the changes in the flow times, the higher throughputs provided some control over the oscillations. However, it was still found that further heating of the walls defining the combustion zone occurred even with the higher throughputs so as to result in an increase in the frequency with lower pressures.